Don't use QHash::unite to merge hashes

QHash::unite can silently turn a regular QHash into a multi hash,
something that is not intended here. Use a regular insert()
instead.

Change-Id: I9244a8553e84eed5367939019347b51491765ea0
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/util/qshadergraphloader.cpp

@@ -136,7 +136,7 @@ void QShaderGraphLoader::load()
         if (prototypesValue.isObject()) {
             QShaderNodesLoader loader;
             loader.load(prototypesValue.toObject());
-            m_prototypes.unite(loader.nodes());
+            m_prototypes.insert(loader.nodes());
         } else {
             qWarning() << "Invalid prototypes property, should be an object";
             m_status = Error;